Scooterotica

hmmm,good advise but in this case i think you need a closer ratio box tbh...li 150 or similar

Yes OK Guys, what I should have said rather than place I live is-- Most of my riding is hilly North Downs, South Downs for e.g.

Anyway staying with the original question, given the power output of the engine, I am not sure 15-46 on a original SX200 gearbox can be improved on to eek better linear power but I would be pleased to hear better options based on 15BHP from my setup

could you not try a lower profile tyre? less hassle!

BHP figure alone means very little, rather it is more about where in the rev range you make that power, i.e. a standard 6 BHP LI150 pulls 5.2:1 4th gear ratio and a 20+ BHP pulls the same 5.2:1 4th gear ratio if fitted with a revvy pipe (see AF's gearing recommendations for TS1s, listed on their website and quoted in Sticky's manual). As such, quoting BHP figures alone means very little when considering gearing.

Don't be surprised in the slightest if your 15 BHP motor struggles to pull the same 5.2:1 that the LI copes with. Your 4.8:1 SX200 ratio is even less likely to pull 4th correctly if you have a revvy/tuned motor.

Adam

Agree with Adan, my Imola (21+bhp) struggled to pull standard sx box, pulled well up to about 50 in 4th but around 50/55 MPH in top, just before it hit the power band, try accelerating at that speed (in all but favourable conditions) and it just struggled, (same on hills) drop it to 3rd, get to about 65 then it would pull 4th.....too much hard work and not a pleasant ride changed to 14/46 (5.1) and it goes like a stabbed rat in any gear at any revs (more fun and easier to ride) but clutch does take a beating (hence me soon changing to an Indain GP200 box and 18/46 = 5.1)
Unless your bike is set up for bottom end torque, as Adam says about state of tune/exhaust etc I think you will struggle to pull 4th for easy day to day riding

Thanks Adam/Padge.

The dyno printout shows 14.95 ft/lbs at up to approx 8k revs. I have thrashed the scooter since I got it back from rebuild and it flies along in all gears and pulls up hills. The only concern I have is--how long will the clutch last..

speed tops out at 68 mph on GPS which is decent

It tops out at 68MPH on standard SX200 box ? that's approx. 6800 revs you say your peak torque is at approx. 8k (quite a revvy set up) therefore I think your bike is over geared as it isn't reaching peak torque in top gear, what sprockets are you running at present ? what's the rest of your set up ?
Even if you change sprockets, there's no way round the jump from 3rd to 4th so I think you will still find it hard (circa 50 MPH) to pull/accelerate in top gear on anything but a flat road

sprockets 14-47
mugello ultimate 200
60 stroked crank (205cc)
variatronic with mapper box
26 phbh with power jet and breath sweet remote
ancillotti exhaust

a change to an Indian GP box (18/47) would give you a similar final drive, a better 3rd to 4th jump and a damn sight easier life on the clutch, those are the reasons I'm gonna change mine